Electrical discharge maching apparatus



v March 29, 1966 w. LQBUR 3,243,567

ELECTRICAL DISCHARGE MACHINING APPARATUS Original Filed May 26, 1961 2Sheets-Sheet 1 lwonrpieg March 29, 1966 w. LOBUR 3,243,567

ELECTRICAL DISCHARGE MACHINING APPARATUS Original Filed May 26, 1961 2Sheets-Sheet 2 INVENTOR.

M72 6 r A 02242 WNW United States Patent 3,243,567 ELECTRICAL DISCHARGEMACHINING APPARATUS Walter Lobur, Clawson, Mich., assignor to EloxCorporation of Michigan, Troy, Mich., a corporation of Michigan Originalapplication May 26, 1961, Ser. No. 112,891. Divided and this applicationMar. 1, 1965, Ser. No.

2 Claims. (Cl. 219-69 This application is a division of US. applicationNo. 112,891, filed May 26, 1961.

This invention relates to electrical discharge machining, particularlyto improved method and machining power circuits therefor.

Electrical discharge machining, sometimes referred to in the art as EDM,spark machining, or are machining, is carried on by passing a series ofdiscrete, localized extremely high current density discharges across agap between a conductive tool electrode and workpiece at sonic orultrasonic frequencies in the presence of a dielectric fluid for erodingthe workpiece.

In electrical discharge machining, the conductive tool is usuallymaintained in proximate position with the workpiece by an automaticservo feed and is advanced toward or into the workpiece as stock isremoved therefrom.

A fluid coolant, usually a liquid, is circulated through the working gapto flush the eroded particles from the gap and is sometimes furnishedunder pressure by a pump through a pattern of holes in the electrode.The defining characteristic of electrical discharge machining is thatthe coolant is a dielectric such as kerosene, transformer oil or purewater and is broken down in minute, localized areas by the action of themachining power supply between the closest points of the tool and work.

Numerous improvements in the art of electrical discharge machining havecaused it to advance from the stage of a laboratory curiosity to ahighly productive machine tool widely used today in the tool room andprduction line. Advanced electrical discharge machining power circuitryutilizes electronic switches such as vacuum tubes for minutely andaccurately controlling the discrete discharges across the machining gap.

This disclosure containsreference to transistors or vacuum tubes orother electronic switches. It follows that wtih proper redesign of thecircuit any electronic switch may be substituted. By electronic switchis meant any electronic control device having three or more terminalsconsisting of at least two terminals acting as a switch in the powercircuit, the conductivity between said power terminals being controlledby a control element within the switch responsive to drive from anexternal control circuit whereby the conductivity of the power circuitis controlled statically or electrically without movement of mechanicalelements within the switch.

An example of the type of machining power circuit representative of thepresent state of the art is shown in Matulaitis and Lobur Patent No.2,951,969, issued Sept. 6, 1960. A machining power circuit such asdisclosed therein,when combined with a power feed of an improved type asshown in Webb Patent No. 2,962,630, issued Nov. 29, 1960, results in amachine having excellent control characteristics and readily usable bymachinists having mechanical skill only and no electrical or electronicsbackground as is typical of the machining trades.

The above machining power circuit contemplates a fixed multivibratorcycle for producing a definite ON- time and OFF-time at a particularmachining tap. I have found that it is desirable to be able to varycontinuously the machine ON-time while maintaining a substantiallyconstant frequency or repetition rate. A machine having thesecharacteristics has infinitesimally adjustable machining current ormachining rate thereby permitting exact adjustment to maximum efficiencymachining conditions at a particular desired metal removal rate andsurface finish or machining gap.

A further objection to the circuitry of Patent No. 2,951,969 is that amachining power circuit capable of extremely high current outputrequired for high metal removal rate necessitates the use of many,sometimes thousands, of vacuum tubes to produce the desired machiningcurrent. Since vacuum tubes are inherently high voltage devicesextremely high power losses result at these machining currents andexpensive equipment and complex electronic circuitry is required. Thecost of operation and consumption of electric power is correspondinglyhigh. Furthermore vacuum tubes are thermionic emissive devices and theiraverage or rated life is approximately 1000 hours. With this limitedlife it can be seen that as the number of vacuum tubes increases, thecost of the basic machine as well as operation and maintenance becomesexcessive.

Accordingly, it is the principal object of this invention to provide animproved machining power circuit having an adjustable ON-OFF ratio orduty factor infinitesimally adjustable throughout the machining currentrange at various selected discharge repetition rates.

Other objects and advantages are disclosed in the followingspecification, which taken in conjunction with the accompanying drawingsshow preferred forms of practicing the invention.

In the darwings in which reference numerals have been used to designatelike partsherein referred to:

FIG. 1 shows a multivibrator circuit employing vacuum tubes having acontinuously adjustable ON-OFF ratio at several different pulserepetition rates.

FIG. 2 shows a circuit having similar operating characteristicsemploying four layer avalanche diodes in place of vacuum tubes as theastable switches and a transistor amplifier in place of correspondingstages of vacuum tube amplification.

FIG. 3 is somewhat similar to FIG. 2 except that controlled rectifiersare employed as the multivibrator switching devices.

FIG. 4 shows a modified form of circuitry employing transistors as theelectronic switches in the multivibra: tor circuit and includingcapacitive means for infinitesimal adjustment of duty factor.

Referring now to FIG. 1, multivibrator plate supply voltages 10 and 12furnish power to the multivibrator cir' cuit containing multivibratorpentode vacuum tubes 14 and 16. The cathode of multivib-rtaor tube 14 isconnected to the negative terminal of supply 10 and the anode of tube 14is connected through plate load resistor 18 to the positive terminal ofsupply 12 which is connected in series with supply 10. Multivibratortube 16 is similarly connected through plate load resistor 20 to thepositive terminal of plate supply 12. A typical cross coupling capacitor22 is connected to the anode of tube 14 and a similar coupling capacitor24 is connected to the anode of tube 16. The opposite sides of therespective coupling capacitors are connected to the opposing controlgrids through selector switches 26 and 28 and operate in conjunctionwith resistors 30 and 32 and rheostat 34 to determine the operating timeconstant of the multivibrator circuit. Rheostat 34 has portion 36connected in the'control grid circuit of tube 16 and portion 38connected in the control grid circuit of tube 14. The adjustable arm ofrheostat 34 is connected through resistor 40 to the positive terminal ofsupply 12 to complete the grid return circuit to the maximum positivevoltage.

A diode 42 connects between the cathode and grid of tube 14 with itscathode connected to the cathode of tube 14 and its anode connected tothe control grid of tube 14, and diode 44 is similarly connected to tube16. Screen current limiting resistors 46 and 48 connect the respectivescreen grids to a common terminal on dropping resistor 50. The oppositeend of resistor 50 is connected to screen grid tap 52. on the main platesupply and resistor 50 in conjunction with capacitor 54 provides afiltered screen voltage tap for the multivibrator.

The output of the multivibrator is taken from the anode of tube 16 andconnects through reference diode S6 and storage capacitor 58 to thecontrol grid circuit of the first stage of amplification. Resistor 60connects to bias sup ply 62 to provide OFF bias for amplifier tube 64during conduction ofmultivibrator tube 16. Grid current limitingresistor 66 connects the control grid of tube 64 to the common terminalbetween the negative side of reference diode 56 and the positiveterminal of bias return resistor 6.0. DC. output terminals 68 and 70provide power for additional stages of amplification or for themachining gap circuit itself which is omitted in this example and maycorrespond to the power circuit of Patent No. 2,951,969. Bias terminal72 may provide bias for the output tube bank or additional stages ofamplification. The anode of pentode 64 connects to terminal 74 which isthe signal output of this multivibrator and first stage ofamplification. Depending on the operating power level, this network mayconnect directly to the machining gap as shown in the above mentionedpatent, or may employ additional stages of amplification prior toconnection to the output tube bank.

The important portion of this circuitry is the network connecting thecontrol grids of the multivibrator to the positive terminal of platesupply 12. The usual return for multivibrator control grids would be forresistor 40 to return to the cathodes of the multivibrator or thenegative terminal of supply 10. rather than the positive terminal ofsupply 12 as is shown in this circuit. By returning this resistor to themost positive terminal, stable operating characteristics are insuredsince the multivibrator tubes pass through the cut-off region much morerapidly when connected to the positive terminal as shown rather than theconventional cathode return. The resistance of limiting resistors 30 and32 and of rheostat 34 must be correspondingly higher to limit gridcurrent and provide the same operating frequency as a cathode returncircuit.

The important grid return connection of this network is that theadjustable tap of rheostat 34 returns the control grid circuits to thefixed positive voltage. As the adjustable tap is moved to the right,increasing resistance 36, it automatically decreases the resistance 38in the opposing grid circuit. It is well known in multivibrator designthat the operating period of a multivibrator may be represented by theformula:

If coupling capacitors 22 and 24 are equal, the formula may, besimplified to:

From this simplified formula it can be seen that as the tap on rheostat34 is moved from one extreme to the other, resistance is similarly movedfrom one grid return to the opposing grid -return thereby maintaining aconstant frequency regardless of the position of the adjustable tap onthe rheostat. Adjustment of the rheostat in this manner of courseincreases the ON-time and decreases the QFF-time while maintaining aconstant repetition rate and provides infinitesimal adjustment of dutyfactor and therefore machining current at any particular selection ofrepetition rate.

4 Switches 26 and 28 are ganged together and are switched to select adifferent repetition rate. Diodes 42 and 44 are provided to shunt thecharging currents of capacitors 22 and 24 from the tube control gridsthereby protecting tubes 14 and 16 and insuring rectangular and fixedamplitude output signal from these tubes.

In this example, six different capacitors are shown in each grid circuitto provide six different frequencies. If these diodes have a higheroperating current than the maximum rated control grid current of themultivibrator tubes, the range of operation of this circuit may beextended materially. By including these diodes in the circuitry,resistor 40 connected to the common potentiometer arm, may be replacedby a direct connection thereby making the effective grid resistance ateither extreme of rheostat 34 equal to resistors 30 or 32. In theabsence of diodes 42 and 44, common return resistor 40 becomes essentialto extend the constant frequency range of the multivibrator since thelower tube voltage drop at the extreme ends of rheostat 34, caused byexcess grid current, is compensated by the resulting lower commonreturn, caused by resistor 40, thereby readjusting operating frequency.

By including diodes 42 and 44, this compensation is unnecessary and theconstant frequency range is extended substantially from a range ofdutyfactor without the diodes but including common return resistor 40 ofapproximately 10% to duty factor for a substantially constant frequencyto a duty factor range with diodes 42; and 44 to less than 1% to morethan 99%. The only limit for minimum ON-time is then the time constantof resistor 20, capacitor 24, connecting to diode 42 in the grid circuitof tube 14 in comparison to the time constant of capacitor 24, resistor32, and full rheostat resistance 36 and 38. In each case in order tomaintain the constant repetition rate or frequency of operation, it isnecessary that the screen grid voltage of the respective tubes and theplate supply voltage each be maintained substantially constant. If theadditional load on the supply furnished by the control grid circuits inone extreme position or the other is negligible compared to the constantamplitude of power resulting from conduction of tube 14 through plateload resistor 18 or tube 16 through plate load re: sistor 20, regulationof supplies 10 and 12 is unnecessary. If additional load is placed onthese supplies varying with ON-OFF ratio at terminals 68 and 70, it isnecessary to regulate the total supply voltage between these two ter-.minals or at least a portion of the voltage presented to the commonpositive return of the multivibrator circuit.

The output coupling circuit of this wide range multivibrator is equallyimportant because a simple coupling capacitor cannot be employed. Inorder to include capacitive coupling to succeeding stages of theamplifier, such as coupling capacitor 56 in the above mentioned patent,it is necessary to confine the variation in ON-OFF ratio to rathernarrow limits. In the above patent itis essential that tube 16 conductfor 10 to .20 percent of the total cycle time in order to transfersufficient power through capacitor 58 and diode. 56 in order to drivethe control grid circuit of that power amplifier.

Effective wide range coupling, is achieved in. FIG. 1 through use ofreference diode 56 and capacitor 58. Reference diode 56 is a DC. voltageregulating. device and maintains a substantially constant voltageregardless of the current flow through the device. A coupling capacitor,on the other hand, is an A.C. device in which equal current must bepassed. in each direction. In the circuit of FIG. 1, capacitor 58.serves to pass the high frequency transients of this circuit andreference diode 56 maintains a substantially constant DC. voltage acrosscapacitor 58 thereby forming a reference voltage at this point.Reference diode 56 and capacitor 58'form a floating D.C. referencevoltage of extremely low shunt or leakage capacity. If a transformer andrectifier network were employed at this point and the necessarily largefilter capacitor that would be required to produce a constant DC.voltage were used, the shunt or leakage capacity between this network,which is floating on the anode of tube 16 and leads 68 or 70, wouldresult in excessive losses byconducting shunt capacitive currents tothese capacitively coupled points. This network is small and of lowmutual capacitance and therefore results in extremely low losses.

A typical value for plate supply voltage between terminal 68 and 70 isapproximately 250 volts. The drop across multivibrator tubes 14 and 16is typically approximately 100 volts during conduction, therebygenerating a signal of approximately 150 volts across the plate loadresistors. In this example, reference diode 56 would be chosen to have avoltage regulation level of approximately 200 volts.

As tube 16 becomes conductive generating the voltage across loadresistor 20 of approximately 150 volts, the negative terminal ofcapacitor 58 is carried downward from terminal 68 by an additional 150volts in addition to the 200 volt reference voltage. This generates anegative bias voltage across resistor 60 and biases the control grid oftube 64 to minus 100 volts at this time and maintains it nonconductiveduring periods of conduction of tube 16.

During periods when tube 16 is rendered nonconductive by themultivibrator action, no drop occurs across load resistor 20 from tube16 and the positive terminal of capacitor 58 is effectively connected tothe positive terminal of supply 12. The negative terminal of capacitor58 is at this instant approximately 50 volts positive with respect tothe cathode of tube 64. The tube 64 is rendered conductive and electronflow from the cathode to the grid of tube 64 develops approximately 50volts across resistor 66 at this time thus completing one complete cycleof operation.

The output terminals of this multivibrator and first stage ofamplification are connected as outlined above to succeeding stages ofamplification or to the working gap itself depending on the magnitude ofoutput power required.

FIG. 2 is also a wide range astable multivibrator circuit employing fourlayer avalanche diodes 76 and 78 in place of multivibrator tubes 14 and16. Anode supply voltage 80 and bias supply voltage 82 furnish DC. powerto the multivibrator circuit and a similar first stage of amplificationis shown in the form of NPN transistor 84. Avalanche diodes 76 and 78are commercially obtainable devices available in the electronicsindustry an dtheir operation as a multivibrator is known. The inventionin this circuit consists essentially of rheostat 86 and the novel returnof control arm 88 to the positive terminal of supply 80. Currentlimiting resistor 90 is connected to the anode of avalanche diode 76 toprovide the minimum time constant in conjunction with capacitor 92 whenreference arm 88 is in the extreme left hand position. Limiting resistor94 is connected in the anode circuit of avalanche diode 78 and operatessimilarly as reference arm 88 is in the extreme right hand position.

The signal output of this circuit is developed in the cathode circuit ofavalanche diode 78 and consists of a signal clipping reference circuitcomprised of reference diode 96 and storage capacitor 98 having theirnegative terminals connected to the negative terminal of supply 80. Thepositive output of this reference network is connected through diode 100to the cathode of avalanche diodes 78. The junction of the anode ofdiode 100 and the cathode of avalanche diode 78 connects to signalresistor 102 having lead capacitor 104 connected in parallel therewith.Transistor bias resistor 106 connects to the negative terminal of biassupply 82. The opposite end of resistor 106 is connected through choke108 to the common junction between network 102, 104 and the base oftransistor 84. Rectangular output signal of tran- 6 sistor 84 isdeveloped across collector resistor 110 and forms at terminal 112 theoutput signal of this multivibrator and amplifier between outputterminals 112, 114, and 116.

The astable multivibrator signal of this circuit is developed by theoscillation produced by avalanche diodes 76 and 78. As an example,assume avalanche diode 76 to be conductive from the negative terminal ofsupply through diode 76, resistor 90, and the left hand portion ofrheostat 86. Capacitor 92 is charged through resistor 94 and the righthand portion of rheostat 86 with little voltage drop occurring acrossavalanche diode 76 during this period of conduction. As capacitor 92becomes charged to the breakdown level of avalanche diode 78, avalancheconduction occurs through this diode instantaneously causing the voltageacross diode 78 to fall to a very low value in the order to 1 to 2volts. At the instant of breakdown of diode 78, the left hand terminalof capacitor 92 is charged negatively with respect to the right handterminal. This negative voltage instantly extinguishes avalanche diode76 causing it to assume a blocked condition. Diode 78 continues toconduct through resistor 94 and the right hand portion of rheostat 86and the negative voltage of capacitor 92 discharges through resistor andthe left hand portion of rheostat 86. Capacitor 92 then charges inopposite polarity assuming a negative voltage on its right hand side. Ascapacitor 92 in conjunction with the steady state D.C. drop acrossnetwork 96, 98, reaches the breakdown voltage of diode 76, the cyclereverses. This astable condition continues at an operating frequencydetermined by capacitor 92 in conjunction with resistors 90, 94 andrheostat 86. As explained in FIG. 1, the total operating time constantof this network remains unchanged regardless of the position of rheostatarm 88 and a constant repetition rate is maintained regardless of theON-OFF ratio or duty factor. As rheostat arm 88 is moved to the extremeleft, diode 78 conducts for a brief interval and diode 76 conducts forsubstantially all of the cycle. As rheostat arm 88 is moved to theright, the opposite condition results in which diode 78 conducts forsubstantially all of the cycle.

Reference network 96, 98, 100 clips the output signal of diode 78 to aconstant magnitude and develops at terminal 118 a fixed amplitudepositive voltage signal dur-' ing conduction of diode 78. This positivevoltage signal draws electron flow from the negative terminal of supply80 through the emitter-base circuit of transistor 84, resistor 102, andlead capacitor 104 to terminal 118 where it is conducted throughavalanche diode 78.

After a brief delay interval, on initiation of this signal a shuntelectron flow occurs from the negative terminal of supply 82 throughresistor 106 and choke 108.

As avalanche diode 78 blocks, choke 108, which is of relatively shorttime constant, sustains electron flow through the base-emitter circuitof transistor 84, bias supply 82, and resistor 106 sharply biasingtransistor 84 nonconductive. As avalanche diode 78 continues in itsblocked condition, bias 82 maintains transistor 84 nonconductive.

Network 96, 98, 100 is provided primarily to limit the drive current oftransistor 84 to a value slightly below saturation drive to minimizestorage time occurring in transistor 84. This network may be eliminatedand transistor 84 may be over driven if storage time in transistor 84 isnot objectionable. Operation in this method results in a longer signalduration across resistor than would be encountered through proper designof network 96, 98, 100. In either case, a substantially rectangularvoltage signal is developed across resistor 110 and produces at outputterminals 112, 114 rectangular pulsating drive signal for successivestages of amplification or for the output network itself.

FIG. 3 is a circuit somewhat similar to FIG. 2 having controlledrectifiers as the multivibrator switches. Controlled rectifiers are fourlayer solid state devices and are similar in many respects to anavalanche diode except that the devices are triggered into conduction bya forward gate signal rather than a voltage breakdown condition.Controlled rectifiers 120 and 122 utilize power from anode supplyvoltage 124 to develop the astable multivibrator signal. Plate loadresistor 126 is connected in the anode circuit of controlled rectifier120 and load resistor 128 is connected in the anode circuit of rectifier122. Cross coupling capacitor 130 directly connects between the twoanodes of the controlled rectifiers and may be compared to capacitor 92of FIG. 2.

The time constant of this circuit is developed by cross couplingcapacitors 132 and 134 which connect the anodes of the controlledrectifiers to the opposing gate circuits. Capacitors 132 and 134 operatein conjunction with limiting resistors 136 and 138 and rheostat 140 tocontrol the total circuit time constant or repetition rate. Operation ofrheostat 140 is similar in principle to rheostats 34 and 86 of FIGS. 1and 2 and forms the basis for the constant repetition rate with varyingON-OFF ratio. As in the above circuits, control arm 142 connects to thepositive terminal of anode supply 124.

Forward gate signal for controlled rectifier 120 is conducted throughdiode 144 and forward gate signal for controlled rectifier 122 isconducted through diode 146 to insure sharp triggering of the devices.Resistors 148 and 150 are provided in parallel with diodes 144 and 146,respectively, to provide high impedance reverse bias during the OFFperiod of each device.

The output signal circuit of FIG. 3 is somewhat similar to FIG. 2 andemploys double anode reference diode 152 connected in the cathodecircuit of controlled rectifier 122 to produce a rectangular controlledamplitude pulse to the base circuit of transistor 154. The conductionvoltage of the diode 152 is generally higher than that of bias 156 andconduction drive to transistor 154 occurs through resistor 158connecting the negative terminal of bias 156 to the base of transistor154 and through limiting resistor 160 and lead capacitor 162. Outputsignal of transistor 154 is developed across load resistor 164 andoutput signal is connected to succeeding stages of amplification byterminals 166 and 168 which in conjunction with reference terminal 170provide the output of this network consisting of a multivibrator andfirst stage of amplification.

In order to review operation of the circuit, let it be assumed thatcontrolled rectifier 120 is conductive thereby developing substantiallythe full voltage of supply 124 across load resistor 126. After the briefdischarge interval of capacitor 130 through opposing load resistor 128,capacitor 130 is also charged in this polarity being negative on theleft side and positive on the right side. Opposite controlled rectifier122 is maintained nonconductive for the duration of the discharge ofcapacitor 132 through resistor 136 and the left hand portion of rheostat140.

As capacitor 132 discharges sufiiciently, the gate of controlledrectifier 122 is rendered slightly positive through diode 146 and thiscontrolled rectifier is triggered into conduction. Triggering ofcontrolled rectifier 122 instantaneously connects the opposite voltageof capacitor 130 across the anode circit of controlled rectifier 120thereby causing it to deionize. At the instant of firing of controlledrectifier 122 substantially the full voltage of supply 124 existedacross capacitor 134 and as controlled rectifier 122 fires, this voltagestored on capacitor 134 renders the gate of controlled rectifier 120negative, blocking diode 144 and causing a minute leakage bias currentto flow through resistor 148. Resistors 148 and 150 are of high relativeresistance to the other circuit re istors and provide suflicient reverseleakage current to maintain each rectifier biased OFF during itsnonconductive period and yet limit reverse current in the gate circuitwithin the. rated limited of the device.

The time constant of capacitor 130 and resistor 126 or 128 is justsuflicient to permit recovery of the respective anode circuit afterfiring of the opposite anode. The minimum time constant of conductionfor controlled rectifier is formed by capacitor 132 and resistor 136with control arm 142 of rheostat in the extreme left hand position. Theminimum time constant of conduction of rectifier 122 is formed in asimilar manner by capacitor 134 and resistor 138 when control arm 142 isin the extreme right hand position. As in the above circuits theposition of control arm 142 determines the relative ON-OFF ratio or dutyfactor while maintaining the repeiti on rate substantially constant.

The output signal circuit of FIG. 3 is quite similar to that of FIG. 2in which a constant amplitude drive pulse occurs at point 172 duringconduction of rectifier 122. The amplitude of this drive pulse iscontrolled by double anode rectifier 152 and is independent of theconduction current of controlled rectifier 122. The minimum conductioncurrent of controlled rectifier 122 must, of course, be sufficient tocompletely drive NPN transistor 154.

During conduction of controlled rectifier 122, drive current fortransistor 154 occurs from the negative terminal of supply 124 throughthe emitter-base circuit of transistor 154, resistor 160, capacitor 162,terminal 172, through the conduction circuit of rectifier 122. A shuntcurrent also occurs from the negative terminal of bias supply 156through resistor 158 and network 160, 162. Excess drive current conductsthrough double anode diode 152.

The conduction voltage of double anode diode 152 is generally largerthan bias supply 156 to prevent a loss of current from bias 156 duringthe normal OFF time. Turn OFF of transistor 154 is achieved from thenegative terminal of bias 156 through resistor 158 and the base-emittercircuit of transistor 154 to the positive terminal of bias 156. Outputsignal is developed across load resistor 164 and is connected tosucceeding stages of amplification through terminals 166, 168 and in amanner analogous to terminals 114, 112 and 116 of the circuit of FIG. 2.It is believed apparent that the drive networks of FIG. 2 or 3 areinterchangeable and either may be employed in each circuit. In FIG. 2,the minimum time constant of recovery can be much smaller therebypermitting a higher repetition rate and diode 100 may be a fast recoverydiode having higher frequency characteristics than either referencediode 96 of FIG. 2 or double anode diode 152 of FIG. 3. With present daycontrolled rectifiers, the recovery time constant is generally in theorder of 10 microseconds requiring that the maximum limits be at leastthat, thereby limiting the maximum possible frequency to 50 kc. with novariaation in ON-OFF ratio or to lower frequency with some variataion inON-OFF ratio. With present. day devices, operation of the circuit ofFIG. 2 can occur with minimum ON times in the order of a fraction of amicrosecond thus permitting much higher frequency response and a broaderrange of operation.

FIG. 4 shows a transistorized wide range multivibrator circuit utilizinga ganged variable capacitor network to achieve constant frequency widerange ON-OFF ratio employing transistors as the multivibrator element.Transistors 174 and 176 utilize power from supply voltage 178 anddevelop signal across resistors 180 and 182 connected respectively inthe collector circuits of transistors 174 and 176. Variable capacitors184 and 186 are ganged together and oppositely phased and operate inconjunction with base resistors 188 and 190 to form the operating timeconstant of the circuit.

The signal output network of FIG. 4 is somewhat similar to FIG. 1 andemploys reference diode 192 and capacitor 194 to form a floating D.C.network connected to the collector of transistor 176. Drive limitingresistor 196 is connected between the negative end of network 192, 194and the base of transistor 198. Transistor 198 9 amplifies therectangular drive signal presented to its base-emitter circuit andproduces an amplified power signal across load resistor 200. Outputsignal of this circuit is taken from terminals 202, 204, and 206 as inFIGS. 1, 2, and 3 and is connected to succeeding stages ofamplification.

It is important in a transistor circuit of this type that thetransistors not be excessively saturated by drive current. Accordingly,it is necessary to provide fixed resistors 188 and 190 to providesufiicient drive current to operate transistors 174 and 176 at or nearsaturation without over driving the respective transistors. For thisreason, the adjustable ON-OFF ratio is achieved through gangedcapacitors 184 and 186. Most transistor circuitry is of relative lowvoltage, low impedance and these capacitors may be formed of theadjustable ceramic disc type developed especially for transistorcircuitry requiring larger values of capacity for the lower resistancevalues typical of transistor circuitry.

Assuming transistor 174 to be conductive, drive current is furnishedfrom the negative terminal of supply 178 through the emitter-basejunction of transistor 174, resistor 190 to the positive terminal ofsupply 178. Transistor 176 is maintained OFF for the duration of thetime constant of capacitor 184 and resistor 188. As capacitor 184discharges and the base of transistor 176 becomes slightly positive,electron flow occurs from the negative terminal of supply 178 throughthe base-emitter junction of transistor 176 and resistor 188 producingan amplified output electron flow through resistor 182. As in thecircuit of FIG. 1, this action becomes regenerative thereby sharplybiasing transistor 174 nonconductive and rendering transistor 176sharply conductive by regenerative multivibrator action.

Capacitors 184 and 186 are of identical maximum size and resistors 188and 190 are of equal resistance. As capacitor 184 is increased incapacitance, ganged capacitor 186 is decreased by a like amount therebyvarying the ON-OFF ratio while maintaining a constant repetition rate ina manner analogous to that of FIG. 1. If it is desired to change thebasic repetition rate, ganged capacitors 184 and 186 are switched in amanner analogous to capacitors 22 and 24 of FIG. 1. In order to preservethe operating characteristics of this circuit, capacitors 184 and 186must be of equal maximum value and as one is adjusted to its maximumvalue, the other is adjusted to its minimum value regardless of totalcapacitance.

The voltage balance between supply 178, reference diode 192, and signalresistor 182 is somewhat analogous to the output circuitry of FIG. 1.For equal drive and bias voltages, reference diode 192 is chosen to havea regulation voltage apporixmately one-half of supply 178. Duringconduction, almost no voltage drop occurs across transistor 176effectively connecting the collector to the negative terminal of supply178. During this interval of conduction of transistor 176, the base oftransistor 198 is biased negative by the voltage of reference 192. Leadcapacitor 208 is connected across resistor 196 to provide a sharpturn-Off current at the instant of turn-Off and accelerated turn-On oftransistor 198. During the OFF periods of transistor 198, thebase-emitter circuit of this transistor blocks conduction, permittingdischarge of capacitor 208. As transistor 176 is rendered sharplynonconductive, lead capacitor 208 is connected to the positive terminalof supply 178 through plate load resistor 182, which is of lowerresistance than limiting resistor 196. For the minute time constant ofcapacitor 208, resistor 182, turn-On of transistor 198 is sharplyaccelerated, thereby producing a sharp rectangular output pulse acrossload resistor 200 connected in the collector circuit of transistor 198.A similar accelerating capacitor may be employed in the grid circuit oftube 64 of FIG. 1 if desired.

Drive current to transistor 198 occurs during OFF time of transistor 176.and conducts from the negative terminal of supply 178 through theemitter-base junction of transistor 198, resistor 1%, reference network192, 194, resistor 182 to the positive terminal of supply 178. The loadof this drive current on the multivibrator is minimized since resistor196 is of higher impedance than resistor 182, thereby producing themajor portion of the voltage drop across resistor 196.

Each of the above four circuits operates throughout wide ranges of dutyfactor or ON-OFF ratio while maintaining a substantially constantrepetition rate of frequency. As outlined above, this operation isimportant in a modern day electrical dischange machining power circuitbut may be utilized in any electronic multivibrator circuit requiring awidely'adjust-able ON-OFF ratio at a substantially constant frequency.

In the above drawings, the DC. supplies are shown as batteries in theinterest of simplifying the disclosure. In actual practice, thesesources of D6. are derived from the secondary of a transformer havingits primary connected to the power source for the machine which may besingle phase or polyphase AC. The secondary voltage is rectified andstored, usually in an electrolytic storage capacitor to form a nearlyide-al D.C. source having very low internal impedance.

Thus, it may be seen that I have shown and described several novelastable multivibrator circuits and a unique method of producing a widelyadjustable ON-OFF ratio while maintaining a substantially constantfrequency and several examples of apparatus constructed in accordancewith the teachings of this invention and employing several differenttypes of electronic switches as multivibrator elements.

I claim:

1. In an apparatus for machining a conductive workpiece by an electrodetool by intermittent electrical discharge across a dielectric filled gapbetween the workpiece and an electric tool, a power source, anelectronic switch having its power terminals operatively connected withsaid power source and said gap for providing machining pulses thereto,an astable multivibrator comprising a pair of electronic switching meansbiased for alternate operation and having an output operativelyconnected to the control electrode of said switch for furnishing pulsesthereto for causing said machining pulses, a pair ofresistance-capacitance networks, each operatively connected to thecontrol electrode of one of said electronic switching means, meansoperatively connected to said multivibrator for selectively presettingits frequency of operation, and means operatively connected to saidmultivibrator for varying the duty factor of said machining pulsesindependently of the frequency of operation of said multivibrator, saidlast-mentioned mean-s comprising means for inversely varying themagnitude of the resistances in said resistance-capacitance networks.

2. In an apparatus for machining a conductive workpiece by anintermittent electrical discharge across a dielectric filled gapIbetween the workpiece and an electrode tool, a power source, anelectronic switch operatively connected to said source and said gap forproviding machining pulses thereto, a multivibrator connected in circuitwith said switch for controlling its operation and including a pair ofelectronic switching means, each having its control electrode connectedto a different resistorcapacitor network, means for simultaneouslyvarying the value of said capacitors for presetting the frequency ofoperation of said multivibrator, and means for increasing the magnitudeof one of said resistors and decreasing the magnitude of the other ofsaid resistors for varying the duty factor of said machining pulsesindependently of the frequency of operation of said multivibrator.

(References on following page) References Cited by the Examiner UNITEDOTHER REFERENCES Mechanical Engineering, Translation from Russian(original published in 1958 in Moscow), pages 1-08, 109

STATES PATENTS 3;, 11 5, 120. Translated Book OTS 60-51089, on. of Tech.Williams 5 Sv=cs., Department of Commerce, Washington, DC

Maillet 219-69 Fefer 219 69 X RICHARD M. WOOD, Przmaly Exammer.

Webb 219-69 X R. F. STAUBLY, Assistant Examiner.

2. IN AN APPARATUS FOR MACHINING A CONDUCTIVE WORKPIECE BY ANINTERMITTENT ELECTRICAL DISCHARGE ACROSS A DIELECTRIC FILLED GAP BETWEENTHE WORKPIECE AND AN ELECTRODE TOOL, A POWER SOURCE, AN ELECTRONICSWICTH OPERATIVELY CONNECTED TO SAID SOURCE AND SAID GAP FOR PROVIDINGMACHINING PULSES THERETO, A MULTIVIBRATOR CONNECTED IN CIRCUIT WITH SAIDSWITCH FOR CONTROLLING ITS OPERATION AND INCLUDING A PAIR OF ELECTRONICSWITCHING MEANS, EACH HAVING ITS CONTROL ELECTRODE CONNECTED TO ADIFFERENT RESISTORCAPACITOR NETWORK, MEANS FOR SIMULTANEOUSLY VARYINGTHE